nitrogen, and to what I may term a "primitive atmosphere," consisting chiefly of nitrogen, with some carbonic acid and vapour of water, and I found that, in both cases, vegetation was remarkably healthy, and even luxuriant, for a lengthened period.

A small plant of Convolvulus arvensis, having its roots in the soil, or in water containing ample carbonic acid and mineral ingredients, vegetated in an atmosphere of pure nitrogen for ten days (22nd June to 2nd July). I then let in carbonic acid, an equal volume, which in twenty-four hours was absorbed by the water to the extent of about one-half; so that the artificial atmosphere, on the next day, consisted of about three volumes of nitrogen and one volume of carbonic acid. In this atmosphere the vegetation became truly luxuriant from the 2nd to the 15th July, and would doubtless have continued much longer had not the experiment been stopped in order to analyse the gas, and because the plant had reached the summit of the apparatus, and was pressing against the glass.

In another similar experiment the residual gas, after fourteen weeks of vegetation, was found to be richer in oxygen than ordinary atmospheric air.1

In these experiments the plant appears to absorb carbonic acid by the roots (as well as the leaves), whilst it evolves oxygen by the leaves; so that, after awhile, the nitrogen atmosphere contains a certain quantity of oxygen and, in time, approaches, or even surpasses, the composition of ordinary air.

1 See APPENDIX for fuller account of this experiment.

CHAPTER V.

Attempts to Define the Primitive Conditions of the Globe from the Results of Modern Research-The Dawn of Animal Life-Effects of the Gradual Increase of Atmospheric Oxygen.

Now, if I endeavour in thought to go back to the primitive ages of the globe, I find that there was probably a period at which the heat was so intense that no compounds could exist. The matter of the Earth then existed in the state of free elements, or, according to my own theory,1 in the state of atoms all identical.

As the temperature decreased, compounds of all kinds were formed according to the laws of affinity; and, finally, there remained, surrounding the solidified surface of the Earth, an atmosphere of nitrogen-a substance which is known to have no tendency to combine directly with other substances.

That there was no free oxygen in this primitive atmosphere is evident from the presence of various oxidizable substances in the primitive rocks of our globe. It is into this primitive atmosphere of nitrogen that plants have poured oxygen, year after year, for countless myriads of

1 Phipson, Outlines of a New Atomic Theory, fourth edition, London, 1886. See APPENDIX.

ages, until it has attained the composition which it has at the present day.

In remote geological periods it may have contained much more carbonic acid than at present; but carbonic acid could never have predominated, from the fact that it would have been absorbed by the waters of the oceans, lakes, and rivers, and from my experiments, which show that even our modern plants can live in an atmosphere of nitrogen, but do not thrive in pure carbonic acid.

I was thus led to the conclusion that the original atmosphere of our globe consisted of nitrogen alone, and that the oxygen of the atmosphere is the product of vegetable life (which must necessarily have preceded animal life). The production of oxygen by the minute unicellular algæ exposed to the light of the sun is a symbol of what took place in the primitive ages of the Earth Carbonic acid must be looked upon as a volcanic product, extensively diffused through the Earth's strata and into the atmosphere and waters. The primitive atmosphere of nitrogen would derive abundance of carbonic acid and vapour from volcanic action, which continued to be very intense long after the Coal flora period, and appears to have gradually diminished from that period to the present time, though it is still very active in many parts of the globe.

Now let me say a few words on the dawn of animal life

1 It might be objected that the plants which first produced atmos pheric oxygen must already have contained oxygen as part of thei tissues. Whence did they derive that oxygen? But I have never sai that plants were the creators of oxygen, only that they were the mean by which Nature has placed free oxygen in the atmosphere of the Earth

OXYGEN, PLANTS AND ANIMALS.

27

I have endeavoured to show that in the earliest ages of the Earth, when life first made its appearance, plants (anaërobics) must have been formed long before animals (aerobics), since free oxygen was absent from the primitive atmosphere. My experiments on vegetation in hydrogen show that free hydrogen could not have existed in this primitive atmosphere any more than it can exist for any length of time in the atmospheric air of our days, without becoming oxidized, and converted into water.

Nitrogen alone, on account of its inert nature, could have formed the Earth's atmosphere in the earliest ages of our planet's history; and previous to the advent of life, this primitive atmosphere was charged with carbonic acid and vapour of water by volcanic action, such as we see manifested to a considerable extent at the present time.

Hence, the earlier vegetative life of the globe developed in an atmosphere devoid of free oxygen, consisting of nitrogen gas, with a certain admixture of carbonic acid and vapour, the whole of the oxygen now present in the air being due entirely to vegetation extending over immense periods of time.

As the ancient plants were evidently anaerobic, it was very interesting to ascertain whether the plants of our present epoch were essentially of the same nature; and my experiments have shown me that they are; also, that they must have preceded animal life.

Animal life has resulted naturally from the gradual transformation of anaerobic cells into aërobic cells, as a

consequence of the changing conditions, that is, the oxygen constantly poured into the air by vegetation.

At what precise geological period oxygen became present in sufficient quantity to allow of animal life might appear an extremely interesting problem to solve; but no such period will ever be determined, because the change must have been exceedingly gradual, and the study of the lower forms of plant and animal life show us that there is no hard and fast line between the two kingdoms. There is no such thing to be discovered as "the first vestiges of animal life."

As the oxygen evolved from the anaerobic cells became slowly and gradually a greater factor in the composition of the air, these cells had to accustom themselves to it, until some became more or less aërobic, and, finally, entirely so, and by their vital functions actually supplied carbonic acid to the air instead of oxygen.

Between green plants, beings which are essentially anaërobic, and the more perfect animals, beings which are just as essentially aërobic, there exists a vast intermediate class which presents, more or less, the characteristics of both; such are the various organized ferments, fungi and bacteria, etc., which represent the gradual transformation of the anaerobic cell into the aërobic cell, under the influence of the gradual change of medium; that is, the constantly-increasing amount of free oxygen in the atmosphere since the earlier geological ages. In the common yeast-fungus we have a familiar example of a cell which has undergone the influence of